It is sometimes desired to provide a tire with a combination of reduced rolling resistance, and therefore improved fuel economy for an associated vehicle, as well as reduced heat buildup, and therefore improved heat durability for the tire itself.
To promote such desirable properties of a tire, it is sometimes desired to reduce the hysteretic nature of various tire rubber components.
Such reduction in hysteresis (e.g. reduction in rubber physical rebound property) of various rubber compositions for tire components may be accomplished, for example, by reducing their carbon black contents.
However, significant reduction in carbon black content of rubber compositions of components in the sidewall region of a tire, whether by simple carbon black reduction or by replacing a significant portion of carbon black reinforcement with silica reinforcement, promotes an increased electrical resistance, or reduced electrical conductivity, of a respective tire component which may significantly increase electrical resistance to passage of static electricity between a tire's bead region and running surface of its tread, particularly as the carbon black content of a rubber composition falls below what as known as a percolation point.
For this invention, a thin rubber strip, or plurality of thin rubber strips, is envisioned which itself (themselves) are of a relatively low hysteretic property (e.g. of a high rubber physical rebound property) and therefore desirably of a low carbon content, yet also have a relatively high electrical conductivity, to promote a path of least electrical resistance between a tire's bead portion and a tire's tread portion, all within a tire sidewall and exclusive of the tire's outer surface.
In practice, a pneumatic tire is desired which has a maximum electrical resistance of 100 megohms (1×106 ohms) between its rubber mounting surface in its bead portion for mounting on a rigid metal wheel rim and its rubber tread running surface (ASTM F1971-99). It can be readily appreciated that if a component of the tire positioned between its bead portion and its tread running surface is not sufficiently electrically conductive, the tire might have an electrical resistance in excess of the aforesaid desired maximum electrical resistance of 100 megaohms. It is appreciated that electricity conventionally follows a path of least electrical resistance. Accordingly, the electrical resistance for the tire between its radially inward bead portion and its radially outward tread running surface can only be as low as the electrical resistance of the electrical path between the tire bead portion and tread running surface. Moreover, insofar as a rubber tire is concerned, it is believed that electricity tends to flow primarily on or within the outer (e.g. visible) surface of the rubber tire. Therefore, for example, if a tire's outer sidewall rubber surface positioned between a radially inward electrically conductive bead component and a radially outward tread running surface has a relatively low electrical conductivity (e.g. an electrical resistance greater than 100 megaohms), then it may desired to provide a means of improving, or otherwise providing, a path of electrical conductivity between the tire bead and tread base layer of a tread of cap/base construction.
Accordingly, it is desired for this invention to provide a tire having an outer, visible tire sidewall layer of a rubber composition having a relatively low carbon black content and an associated relatively low electrical conductivity (relatively high volume electrical resistivity property for the rubber composition itself), because of its relatively low carbon black content, in order to promote a relatively higher 100° C. rebound physical property for the visible tire sidewall rubber composition (because of the reduced carbon black reinforcement) as compared to such a rubber composition with a significantly higher carbon black reinforcement content. In practice, such relatively low electrical conductivity for the rubber composition itself might be represented, for example, by a relatively high electrical volume electrical resistivity according to ASTM D 257-98 of at least 1.0×108 and particularly at least 1.0×1010 ohm-cm which can be presented as 1.0E8 and 1.0E10 ohm-cm, respectively.
In one aspect of the invention, in order to provide a path of increased electrical conductivity, and consequently a path of least electrical resistance, for the tire sidewall (to counteract the significantly low electrical conductivity of the outer sidewall layer), a strip of a rubber composition having a contrastingly relatively high electrical conductivity (a relatively low electrical volume resistivity) is provided as a new tire sidewall component, and as a departure from past practice, which extends internally within the tire sidewall (and therefore axially inward from the outer tire sidewall layer) from a carbon black-rich rubber chafer component in the tire bead region radially outward to a carbon black-rich rubber tread base of a rubber tread of a cap/base construction, wherein both of the chafer and tread base rubber compositions have a relatively high electrical conductivity (relatively low electrical volume resistivity) because of their relatively high carbon black reinforcement contents.
Historically, the providing of one or more strips or layers of an electrically conductive material, such as for example, a rubber composition, within a tire construction to reduce, or eliminate, static electrical effects caused by operation of the tire was recognized at least as early as 1940.
U.S. Pat. No. 2,339,546 relates, for example, to a non-static tire which contains an element as an electrically conductive rubber composition of which one terminus of the element is positioned in the tire bead portion and adapted to be in electrically conducting relation with a tire rim and the element extends on the outside of the tire body and around the tire shoulder to the other terminus of the element in the face of the tire tread with a protective sidewall covering a portion of the outer surface of the element.
U.S. Pat. No. 2,342,576 relates, for example and at least in part, to providing one or more electrically conductive rubber strips beneath an electrically relatively non-conductive tread. Such strip may in a form of a circumferentially rubber strip which contains a significant electrically conductive carbon black content such as, for example, an acetylene black and/or certain channel blacks. The electrically conductive strip terminates short of the tire beads or rim-contacting portions of the tire.
U.S. Pat. No. 2,641,294 relates, for example, to a passenger radial tire of which at least a portion of the outer surface of a sidewall contains an electrically conductive carbon black and has a volume resistivity of not more than 104 ohm cm.
U.S. Pat. No. 5,173,135 relates to, for example, a radial passenger tire with a sidewall having a volume resistivity of not more than 104 ohm cm which it considers as being relatively electrically conductive.
British Patent Publication No. 544,757 relates to, for example, a tire having an electrically conductive path extending substantially from the bead region of the tire to the road contacting surface of the tread. The electrically conductive path is provided by an electrically conductive rubber cement applied to specific portions of the tire functions to discharge static electricity. The rubber cement contains a high acetylene black content.
Japanese Patent Publication No. 52/47202 relates to, for example, a static-dissipating rubber tire in which the tread rubber has an electroconductivity of 10−3 mho or better and a portion of the sheath rubber of the tire sidewall has an electroconductivity of 10−8 mho or better to form a static-dissipating path for static to pass from a wheel rim to the tread surface in contact with the road.
In the description of this invention, the term “phr” relates to parts by weight of an ingredient per 100 parts by weight of rubber, unless otherwise indicated.
The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” are used interchangeably unless otherwise indicated. The terms “compound” and “rubber composition” may be used interchangeably unless indicated. The term “carbon black” is used to refer to rubber reinforcing carbon blacks unless otherwise indicated. Exemplary rubber reinforcing carbon blacks may be referred to, for example, in The Vanderbilt Rubber Handbook (1987) on Pages 414 through 417.